A computing center provides access to a plurality of physical computing machines. Examples of computing centers include data centers or data warehouses, computer clusters, and server farms. Generally, a computing center is designed for a varying level of use such that the typical use is within the computing capacity of the center. When usage is below capacity, idle machines can be operated at a reduced power or even shut down completely. For example, in some computing centers, each physical computing machine is capable of hosting virtual machines. The virtual machines provide various computing services, typically accessible via a network. Each physical machine can typically host multiple virtual machines. When usage is below capacity, some physical machines are idle but consuming energy; these machines can be powered down. A request for a new virtual machine can be handled by an active physical machine if the physical machine has availability, otherwise another physical machine may need to be activated, e.g., powered up. A delay caused by activating a physical machine can create an undesirable lag time in satisfying a request for a new virtual machine. These delays can be avoided by maintaining idle physical machines. However, expending energy on an idle physical machine can be wasteful and expensive.